Half-tone memory tube



June 17, 1958 v F. H. HARRIS HALF-TONE MEMORY TUBE 6 Sheets-Sheet 1Filed May 16, 1952 FRANKLIN H. HARRIS BY Q6 6&7@-

ATTORNEY F. H. HARRIS HALF-TONE MEMORY TUBE June 17, 1958 6 Sheets-Sheet2 Filed May 16, 1952 INVENTOR FRANKLlN H. HARRIS ATTORNEY 6 Sheets-Sheet3 PR I MARY BEAM CURRENT R E Z INVENTOR FRANKLIN H. HARRIS BY 95MATTORNEY June v17, 1958 Filed May 16. 1952 D E D. R 5 A S B O \II NMMMIO EOT I EBL T WDW T b EN( BAC E m NOC E E RRL ENE 2 F I .S FELD I `DEFTcl. A T WV I I 0 NEN OIn V NGE OV ERM 2 a IUE I OOL PSE 4 T h h3 O O Oh 2 I n .nv w 2 2 .T .T E .n Z hl Nz .aS-q ghomjm 0.". PZmDo .mz -IPOTENTIAL DIFFERENCE BETWEEN CONDUCTING MEMBER AND HOLDINGCATHODEJVOLTSI June 17, 1958 F. H. HARRIS HALF-TONE MEMORY TUBE l l 6sheets-sheet 4 Filed May 16. 1952 @M 295mm d N. 295mm INVENTOR FRANKLINH. HARRIS ATTORNEY June 17, 1958 F. H. HARRIS HALF-TONE: MEMORY' TUBE 6Sheets-Sheet 5 Filed May 16, 1952 INVENTOR FRANKLIN H. HARRIS ATTORNEYF. H. HARRIS 2,839,679

HALF-TONE MEMORY TUBE 6 Sheets-Sheet 6 BY Q WL# ATTORNEY June 17, 1958Filed May 16. 1952 sooo/LL AMP. IN.2

POTENTIAL DIFFERENCE BETWEEN CONDUCTING MEMBER AND GATHODE United StatesPatent Gflce 2,839,679 latented Julien, ross -This invention relatesgenerally to cathode Vray storage systems, and more specifically to acathode ray storage v system capable of recording, storing,'andreproducing, for an indefinite numberof repetitions, amplitude gradationinformation signals. This invention also specifically relates to anarrangement whereby a relatively high output current for readingpurposes `may be 'obtained from a cathode ray storage system.`

Cathode ray tubes are well known in the prior art which will perform thesequential functions of an information device, namely recording,storage, reproduction and erasure of an information signal.These'electron discharge tubes operate upon the vprinciple oftranslating received information into charges and coincident potentialsupon the surface -of VIan insulating material and subsequentlyrecovering this information by exploring the surface with a focussed'electron beam. As a 'result of the extremely high impedance offered byan insulator to the flow of current to and from a ,charged area, in theabsence of a reproduction operation the charges and coincidentpotentials initially impressed upon the dielectric will persist withoutsubstantial deterioration over a period of time during which theinformation which they represent may be made available. When elicitationof the information stored Vis accomplished, however, by scansion of theinsulating surface With an electronreading beam, the information Vbeingrecovered either as a secondary emission current from the VdielectricorV as capacitively induced current due toV a shift in potential of theinitially impressed' potentials, in the prior art theV informationbearing charges and potentials are progressively destroyed withvsuccessive repetitions of the reading operation. Thus in the prior artinformation reproduction can only be effected by destructive reading.

`In the copending application of Andrew V; Haefl', Serial No. 768,790led August l5, 1947, there is taught as a departure from the prior art,the employment of a holding Abeam which counteracts the tendency of thevcharges and coincident potentials to deteriorate because of destructivereading. Briefly, in the tube disclosed by Haeif a pattern of chargesand potentials is impressed upon a continuous dielectric surface, eachcharge and its coincident potential representing a single resolvableinformation item. Thereafter the surface of the dielectric is exposedeither continuously or intermittently to a holding beam of electrons.This holding beam in falling' on the dielectric target has a firstephemeral action ofY driving individual areas, coextensive with theinitial charges representing information items, upwards or downwards inpotential to one of twopotential limits, the course taken correspondingto the sense ofthe initial potential of the area as measured relative tola critical potential. The holding beam also has a second enduringaction of stabilizing the information pattern as so modified bycontinuously regenerating the charges and potentials at the respectivelimits to which they have been driven. Binary information onceVestablished upon the Surface of the dielectric may thus be reproduced anun` 'teristics of the' Vtarget under bombardment by Va holyi 2 Y limitednumber of times without destruction of the pattern, the tendency towardsdeterioration occasioned `by the action of the reading beam vbeingcounter-acted by the self restoring actionv of the holding beam.

Since each charge represents a single vresolvable item of information,andv since each area loccupied by an initial charge is driven by theholding beam to one 'of two potential limits, the Haef device is notyadaptable"to the faithful reproduction o f received information signalsrepresenting intermediate tones between black and w te without specialexpedients seriously limitingtheamiint of information handled; Y

Further, 'in the prior art devices the reading b.

m Produces a shift Vin potential of the information a as scanned, duringthe course of which'shift a utilizable reading current of secondaryemission electrons isprof duced. ln these devices, however, themagnitude 'of reading current obtained Ais onlyv a relatively smalllper? centage of the primary current of the reading beam.

It is `therefore an object of this invention to provide an arrangementwhereby an amplitude gradation informa-V tion signal may be stored in acathode ray storage device and may be reproduced therefrom an unlimitednumber of times. It is a yfurther object of this invention to providearrangement whereby in a cathode ray storage ydevicea reading outputcurrent may be obtained vof .relatively high magnitude compared to priorart devices.r :Y

It is a further object of this invention to providestolfage targets fora cathode ray storage device whereby'received information signals havingamplitude gradatioiis may be stored and be repetitively reprdLICed,Without deterioration, by va relatively` high magnitude output Vcur!rent compared yto prior art devices. i It is a further object of thisinvention to provide a' method whereby amplitude gradation informationsigl' may' be recorded, stored and" repetitivelY Ieprod'uA d withoutdeterioration at a relatively high magnitudeo'ut-` put current comparedto prior art devices. Other and further lobjectsand features of thepresent invention will become apparent upon a careful consideration' ofthe following description when taken together with the accompanyingdrawings which illustrate tyl? l features of the invention and themanner in which invention may' Abe considered to operate.

In the drawings. Figure l is a diagrammatic representation of an`illustrative embodiment of the present invention and operating circuitstherefor; I v

Figure 2 is an enlarged representation of a portionof the surface of anillustrative type of target, utilizing'a back plate, as viewed from thecathode end of thefstor'age tube;

Figure 3 is an enlarged representation of a portion of the surface of aVvariant type of target, utilizing 'a rn V h l and Figure 3a is across'sectional view of Figure; Figure 4 is 4a graphical representationof Vthe ,current characteristic of an isolated dielectric particle.under bombardment of a` beam Vof electrons; Figure 5 representsgraphically the current characng beam of ,electrons at various selectedvoltages;

Figure 6 is a representation, as shown by a greatly enlarged crosssectional view of a portion of ya mesh ,type target, of electricalconditions in the vicinity `of Vthel target `surfaceifor various holdingvoltages; *i l Figure 7 is a greatly enlarged elevation o f a dielectriccell embedded between the wires of a mesh type target, and isillustrative of the different capacitance zonesv within a singledielectric cell;V

Figure 8 is a cross secti rralyiew of Figure 17 tak n -Figures 9-10'are"graphical representations of modes ofoperation of the tube toproduce negative writing;

Figure 11 is a graphical representation of a mode of operation of thetube to` produce positive writing; and Y Figure 1'2 is a representation,as shown by a greatly enlarged cross sectional view of a portion ofthemesh type' target, ofelectrical conditions in the vicinity of a targetsurface produced by a reading beam.

Briefly Vthe objects of invention set forth above are achieved byemploying in la holding beam type of storage discharge device a storagetarget having, by'way of contrast tothe homogeneous exposed target facesutilized in theprior art, a heterogeneous face exposed to theelectron`beams,"this face consisting of a member of electrically'conducting material and a myriad of separated dielectric masses`dispersed throughout the surface of the conducting memberV insufficient concentration so that a considerable number of `thesedielectric masses appear within. an area approximating in dimension thespot of a focussed electron beam. As 'will be explained hereafter, byvirtue of this arrangement potentials, representing information items,when once impressed upon the mentioned areas by the writing beam willnot be'altered in their values by the action of the holding beam and yetwill be retained thereby. Amplitude gradation informationitems may thusbe stored for an unlimited period and be reproduced an unlimited numberof times. Further, by virtue of this arrangement, reading signalcurrents may be obtained without disturbance of the information patternand having magnitudes approximating the value lof reading'beam primarycurrent.

lReferring now to Figure 1 there is shown an electron discharge storagedevice or tube generally designated by 20,V comprising an evacuatedenvelope 21 of glass brother material, with an after section 22,midsection 23 land forwardsection 24. Positioned in the rearward regionof after section22 and within envelope 21 are located three cathodes,namely writing cathode 25, holding` jcathode 26 and reading cathode 27,the aforementioned cathodes 'being capable of heating to anelectronemitting temperature by means not shown. Cathod'es 25,"26 and 27are separately connected to 'the negative terminals of variablevoltagesupplies 28, 29 and 36, respectively, lthe positive terminals of whichare conriected to ground. Cathodes 25, 26 and 27 are thus alwaysmaintained, with respect to ground, at a negative adjustable directvoltage. Holding cathode 26 in addition may have an alternating voltageimpressed thereon by closure of a switch 31 which completes a connectionto alternating signal source 32.

'Y In alignment `with and forward of writing cathode 25 in the ordernamed, in mutually spaced relationship are positioned a grid 33, andaccelerating and focussing assembly34 and a deliecting assembly 35, `theaforementioned elements comprising a conventional electron gun. Portionsof assembly 34 are connected to `the maximum potential terminal 35 4of ahigh voltage supply 36, 'the negative terminal 37 of which is connectedto ground. Assembly 34 is vthus maintained at a positive potential withrespect to ground. Electrons emitted from writing cathode 25 areresolved by accelerating and focussing electrode assembly 34 into lawriting beam of small cross sectional diameter and high intensity, theintensity of which may be modulated by information signals, particularlyof the amplitude gradation type, originating with an information signalsource 38 and impressed upon grid 33 in the form of varying voltages.During passage through deflecting assembly 35 the writing beam may besubjected to further control, this time as to assumed direction, byvoltages originating with horizontal sweep s-ignal source 39 andvertical sweep signal source 40 and impressed upon horizontal deflectingplates 41 and verti-` cal deecting plates 42 respectively. By properselection andtsynchronization of the horizontal andvertical sweepslgnals, the writing beam may be calnised to trace `out desired scansionpatterns upon a cross sectional area of 'forward"section 24; Onescansion"`pattern found suitincremental vertical step at the left handpattern border,

preparatory to sweeping out the next odd horizontal line.

Similar to the arrangement for the writing beam, in alignment with andforward of reading cathode 27 in the order named, in mutually spacedrelationship are positioned a grid 43, an accelerating and focussingelectrode assembly 44, and a deecting assembly 45, the elementsmentioned forming another conventional electron gun. Accelerating andfocussing electrode assembly 44 is Amaintained at positive potentialwith respectA to ground `by a connection to maximum potential terminal3S. Electrons emitted vfrom reading cathode 27 are resolved 'by assembly44 into a reading -beam Iof high intensity and approximately the samecross sectional diameter as the writing beam. `l`he reading beam thusproduced may be deected in directionby signals originating withhorizontal sweep signal sour-ce'46 and vertical sweep signal source 47and ,impressed as voltages upon horizontal deflecting plates 4S andvertical deflecting plates 49 respectively. Thus, by selection of properhorizontal and vertical sweep signals the reading beam in a mannersimilar to the writing beam may be caused to sweep out desired scansionpatterns on a cross sectional area of forward section 24. Further, theintensity of the reading beam may be controlled by voltages limpressedon grid 43. `In the present embodiment grid 43 may be connected Vby twoposition switch 50 to cathode 27 for ordinaryl reading or alternativelyto radio frequency signal source 51 for a purpose to be later more fullydescribed( Turning to a consideration of the holding beam electronV gun,in 'alignment with and forward of holding cathodeV 26 in theorder-named, in mutually spaced relaltionship are positioned a Vgrid 52,an accelerating anode 53, al focussing electrode 54Y and anotheraccelerating anode 55;' Anode 53, electrode 54 and anode 55 togetherresolve electrons emitted from holding cathode 26 into a [holdin-g beam.Focussing electrode '54 is maintained at a negative potential withrespect to ground byr a` connection Vto cathode 26. Accelerating anodes53 and 55 are maintained at `a high positive potential with respect Vtoground by connect-ions to maximum potential terminal 35. Electric eldsare thus established between focussing electrode `54 and'n acceleratinganodes 53 and 55, which fields in effect form a powerful electron lens.The converging action of this lens produces a cross-over and subsequentwide divergence of electron rays originally mildly diverg-ing from theholdingubeam axis. As a result, the holding beam electrons in forwardsection 24 are uniformly diffused at a Ilow area density across a majorportion of the tube bore.

The holding beam intensity may 'be controlled by the magnitude ofvoltage applied to grid 52. In the embodiment shown, by means of twoposition switch 56, grid 52 may be either directly connected to cathode26, or may be connected to `the negative terminal of bias supply 57, thepositive terminal of which is connected 'to cathode 25. The lformer-andlatter connections produce on and off conditions-for the holding beamrespectively.

For convenience and space economy the writing, holdi ing and readingelectron guns may be supported in a tri- Collector `electrode '58,rn;1ybeA conveniently composed` of .a layer ofconduotingmaterial,ltypicallyaquadagdeposited on the inside surfaceof envelope 21. Forproper shielding, collector electrodeis. maintainedat a' positivepotential with respect to ground by a connection to terminal 35 of highvoltage supply 36.

Coaxially spaced forward of collector electrode 58 and insulatedltherefrom is located a similar ring or collector electrode 59 ofsmaller axial extent. Ring 59, which also may be conveniently composedof a layer of aquadagl deposited on envelope 2l, is maintained ata lowerpotential to ground than collector electrodef by a connection to amedian terminal 6i) of high voltage supply 36.` Ring 59 performs thefunction of parallelization of the rays of the various electron beamspassing therethrough, with the result that each ray :of a given electronbeam impinges at the same angle, upon a subsequent cross sectional areaofthe tube.

Within section 24 and forward ofring 59, a plurality of members 6decooperate to support a target 61. concentric with the axis of tube 20.`Target 61 is charac# terized by a substantially 'planar face` exposed tothe electron beams, which face is normal to the tubevaxis, and occupiesthe larger part of thetube bore atthat location. The entire `area ofthetarget face is subject to bombardment by the electrons ofthe diffusedholding beam.

Considered from the point of view ofits electricalproperties, target 61is constituted of two distinct portions, namely `a flat,continuously-connected, conducting member 62 coextensive with the target6l` itself, and an' ag; gregate of dielectric material appearing as amultitude of minute, mutually separated dielectric massesdistributedthroughout the exposed target face. While Figure l shows dielectricmasses 63 Vdeposited upon conducting member o2, alternatively the massesmay partially oc'- cupy interstices in conducting member 62. Regardlessof the particular structure adopted, the eiective facing of the targetas viewed by the electron beams presentsa heterogeneous appearance,being a composite'jof a myriad mutually spaced `dielectric materialexposures ntermingled with exposures of the `conducting member.

Conducting member 62 of target 61 is connected by lead 64 to one end ofan impedance 65, typically a resistance, the other end of which is`connected to ground. Under bombardment by the various electron beamsconducting member 62 suffers a net loss or enjoys a net gain ofelectrons.

ln the case of a net gain of electrons a negative current (equivalent topositiveVV electron ow) will be produced, flowing from ground throughimpedance 65 and lead 64 to conducting member 62, alongthe particularimpinging beam (in a direction opposite to electron 'movement) to thecathode from which the beam emanates, through the associated variablevoltage supply, and back to ground. In the case of a net loss ofyelectrons from secondary emmission, some ofthe secondary emittedelectrons will travel from the surface of conducting member 62 to themore positive collector electrodes 58 or S9. A positive current therebywill be produced owingV from conducting member 62 through lead 64',impedance 65, high voltage supply 36, collector' electrodes 5S `or 59,and back to conducting member 62' alongV a path opposite in direction tothat of the secondary emitted electrons. With either type phenomenon,coeval with the ilow of current .a voltage will `be created between theupper terminal of impedance 65 and ground. When the target face issubjected to the scanning action ofl the writing or the reading beam,the voltage generated thereby' across impcdance 65 is of a lluctuatingnature which may be coupled to utilization device 66 by a condenser 67connected between the input of device 66and the ungrounded side ofimpedance 65. When Vdesired, for a purpose to be later more fullydescribed, by closure 4of switch 68, a filter 69 tuned to the frequencyof radio frequency signal source 51 may be connected betweenthe input ofutilization device 66 and ground.

It will be understood of course that electromagnetic:`

Ground to terminale35 r +1200 volts.`

Ground to terminal 60 |200` volts:

Ground to cathode 25l -600 Volts.-

Ground to cathode 26 -190 to 210 volts:

Ground to cathode 27 19040-210 volts.

Writing beam current at target-- l().microamperes.l

Reading beam current at target l-S microamperes:

Holding beam currentiat target-.. microamperes-f.

Size of focussed reading and n i writing beam spot ortargeL- 40 milsVdiameter (.04

inch).

,ConsideringV now in more detail the target 61 ofelec tron dischargestorage device 20, Figure 2` representsin' greatly enlarged crosssection an elevation of a` plate type' of target :as viewed from the`cathode end of tube 2li.I That portion of Figure 2 designated as 62arepresentsav member comprised ofV electrically conducting material', themember having a substantially Yplanar viewed surface. Ir'ronev varietyof plate target, member 622i consists of a sheetV of mirror finished-valuminum, thi'sf'particul'ar rna-te;'Y

rial being selected because ofY its high ,andA stablefsec'iond-aryemission characteristics.Y tion of the information pattern. impresseduponY the target Y is desired, alternatively member 62a may -consistofathin, transparent, conducting film, producedupon a transparent: glassbacking, for example,A asdiselosed in Patenti 2,064,369 granted to O. H.Biggs on December 15, 193.6.

Regardless of the composition of member 62a, those portions of Figure 2designated. as63a represent. massesof dielectric material takingthe'form ofrandonr size-.para

ticles evenly distributed, in adhering relationship,- overIk the surfaceof member 62a, in sufficient. scarcity of num-- ber so that most of theparticles are mutually separated. Within the aggregate, the individualparticles of dielectric vary in size below an upper limit meandiameten;` typically 10 microns. Because of their smallY size. and closeproximity many hundred dielectric particles are-lcon centrated in agiven focussed beam spotarea. Particles 63a m-ay be composed of anysuitable dielectric material;

In practice a phosphor having the chemical formulaV (ZnSzAg) anddesignated by the Radiov Manufacturers. Association as P-ll has beenfound to be satisfactory.

The plate type target described above may be'manuf factured in thefollowing manner. The conducting. member 62a is first chilled below thedew point so thatV a thin film of water condenses on itsv surfage.Member' 62a is then placed in an enclosed space into which is. blown aquantity of dielectric particles 63a` which scatter to form a nelydispersed cloud. The airborne particles are allowedv to settle upon thesurface of the conducting,- member 62er untilV a proper concentrationhas been reached, at which. time member 62a. is removed and the. waterlilm hitherto clinging to its face is driven oli". During theevaporation of the film a capillary elect is produced which capillaryaction forces the dielectric particles into intimate contact with member62a, thus causing the dielectric particlesto adhere strongly'to member62a.

While the'plate type of targetthas'proved satisfactory in the 'practiceof the present invention, as a result ofthe random sizeA andunpredictable distribution of the Vdielectric particles within aunit'area, which term is used y to designate a subdivision of the targetspaceaequalrinl WhereY visual obser-va-v 7 ties of `a unit area 'oftarget face vary'slightly from one unit area to another. Thus'from theelectrical point of view, the surface of the target is to a minor'degreeirregular, which roughness may inject a certain amount of noise into theperformance of the storage device.

Referring now to Figure 3, the figure represents an enlarged elevation,viewed from the cathode end of tube 20, of a portion of a mesh typetarget, which mesh type target by Way of comparison to the plate typetarget hitherto described,-exhibits, electrically speaking, a facecomprised of highly uniform unit areas. Conducting member 62b in thiscase comprises a finely wovenfmesh, typically 230 meshes to the inch, ofa material having good secondary emission characteristics, typicallylstainless steel. In contrast to the usual mesh structure wherein thestrands, viewed edgewise, follow sinusoidal paths, as is more clearlyshown in Figure 3a, the wires of my mesh, viewed edgewise, extend in astraight line except in crossing point vicinities where they are exedunder an intersecting wire. All of the wires are thus for the majorportion of their length tangential to a plane surface lying above themesh.

, The interstices of the woven mesh are partially or fully closedby acoating of particles of dielectric material, the sum of which particleswithin an interstice will hereafter be denominated as a dielectric cell.Cells 63h occupy spaces within the interstices preferably below thelevel of the aforementioned tangential surface. VThose parts of Figure 3designated as 64b represent cavities in the dielectric cells, thecavities being produced incidental tothe manufacture of the target. Thepresence or ab sence-of cavities 64b is a matter of indiierence, sincedielectric material if located in the central portion of the cell wouldhave a negligible effect upon the operation of the device.

. While dielectriccells 63b are considerably larger in size thandielectric particles 63a shown in Figure 2, the cells 63b having atypical dimension along a side of 2 mils, the cells are still sucientlysmall that a considerable number, typically 67, will be centered withinan information unit. As a result of the orderly array of cells withinthe interstices of the woven mesh the electrical properties of thetarget face, considered in terms of information units, will be uniform.

In practice a satisfactory mesh target has been made from a 230 meshstainless steel screen with its interstices lled by (Zn2SiO4zMn)phosphor, designated by the Radio Manufacturers Association as P-l.

' The mesh type target may be manufactured in the following `manner. Anordinary nely woven mesh is passed between two polished rollers whichatten its surface. A suspension of dielectric particles in acetone,having dissolved therein a plastic acetate binder, is thensprayedragainst the back side of the mesh at an acute angle to itsplane. By depositing the dielectric material on the screen from an acuteangle, the dielectric particles build up within the interistices ratherthan accumulating upon the back surface of the wires where theirpresence would be ineffective. During Vthe spraying action the mesh isrotated around an axis normal to its surface to insure an evendispersion of dielectric material. The dielectric particles accumulatedin the interstices will be bonded together and to the wires by theacetate binder upon evaporation of the acetone. The front side of themesh is utilized as the storage surface of the target.

Turning now to principles generally applicable to electron dischargestorage devices, it' is obvious that the smallest subdivision of atarget face capable of resolution by a focussed electron beam, and alsothe subdivision at which resolution inevitably takes place, is an areaof the target face equal to the spot size of a focussed electron beam;which area has hitherto been described as a unit area. To the primaryelectron beam each unit potential whichmay be termed respectively as amean charge and a mean potential, and which 'have the same effects Vasuniform charges and potentials distributed evenly throughout theaun'it'area. While the charges and potentials throughout a given unit areawillappear to the electron beam as a homogeneous entity, diiferencesbetween mean potentials successively scanned are easily detectable. Itis thus seen that each mean potential displayed by a unitarea'represents a Ysingle and separate information item.

It willalso be obvious that if within each unit area the storage surfaceis electrically irregular a number of subareas may exist within eachunit area relatively heterogeneous in charges and potentials, althoughthis variation is not seen by the electron beam. In such a case theresolvable mean charge'and coincident mean potential of a unit area arethe resultants, of the plurality of subarea charges and potentialswithin the unit area.

For purposes'of Vclear distinction in terminology, as to this tine-level of phenomena which actually exists though it is rresolvable tothe electron beam, the sub areas will be termed elemental dielectricportions, the charges and potentials assignable to each portion, thesimple charges and potentials, and the entirety of the various simplecharges and potentials throughout the storage surface as the charge andpotential patchwork` respectively. By way of comparison, analogousfeatures at the level of phenomena resolvable to the electron beam arethe unit area, means charge, mean potential andthe charge and potentialpatterns. Transposing these latter mentioned features into terms ofinformation which they represent, the storage surface acts as aninformation accumulator consisting of a bank of information units, eachof which exhibits an information item, the conguration of theseinformation items being an information pattern. i

Considering more specifically the modes by which th present tube may beconsidered to operate it is necessary to consider the phenomenon bywhich information initially established on the target may beindefinitely preserved. Figure 4 represents graphically the variance ofcurrent flowing to the surface of an isolated elemental dielectricportion in dependence upon Vthe potential between said portion and abombarding electron source. Since' the elemental dielectric portion isspecified. as isolated, the values of the current curve of Figure 4 donot include the current effects resulting from the interactions betweenadjacent dielectric portions of thc target face, which interactionspractically occur. Primary electrons emitted from the source andabsorbed by the dielectric surface are considered to contribute anegative current to the dielectric portion. Secondary electrons,produced by bombardment of the surface by the primary beam and escapingfrom the'surface to collector electrodes 58 and 59 or to conductingmember 62, are considered to furnish a positive current to thedielectric portion. The ordinate of Figure 4 thus represents the sum oftwo simultaneous currents or the net current to the surface of thedielectric portion. The abscissa of Figure 4 represents the potential ofthe bombarded dielectric portion with respect to the electron sourcewhich is considered to be at zero volts. Point c on theabscissa ofFigure 4 represents the state of the conducting member of the target,which member is maintained atVc volts.

Assume now that an elemental portion of dielectric can be given at willany potential along the abscissa of Figure 4, and that the portion isbombarded for a momentary interval of time with electrons vfrom thesource, which electrons necessarily have an energy equal to that of theimpressed potential. As the potential of the portion is raisedpositively from zero volts, initially primary electrons will be absorbedby the dielectric surface, resulting in a negative net current as shownby the current curve between points'h and a. Bombardment of thedielectric surface by the` primary beam electrons, however.

produces secondary emission electrons, the numbers of which increase ata faster rate with the increasing potential than the number of primaryelectrons absorbed. At point a on the curve conditions are such that foran incremental increase of potential, the additional secondary electronsescaping from the surface as a result of the increase, exceed in numberthe number of additional primary electrons absorbed.

The current curve thus becomes positive going, crossing the zero line ato, `at which point the total number of secondary electrons escapingfromv the surface of the dielectric equals the total number of primaryelectrons absorbed by it The potential V at which cross-over point ooccurs is known as the critical voltage, and it is determined only bythe type of dielectric material.

When the potential is increased somewhatbeyond the value of V0, thecurrent curve enters the positive region in which the number ofsecondary escape electrons exceeds the number of primary electronsabsorbed. From point o up to a point b the current curve is stillpositivegoing. At point b, however, the net current starts to decreasewith increasing potential. The explanation for this change of trend isas follows: For potentials below point b, secondary electrons onceemitted from the dielectric surface are drawn away from it to the higherpotential rings S or 59 or the higher potential conducting member of thetarget. At point b, however, the potential of the dielectric portion isapproaching the potential of the conducting member. Conducting member 62begins to exert a suppressive effect upon the emitted secondaryelectrons. As a result considerable number of secondary electronsinstead of escaping the rings S3 or 59 ultimately fall back on thedielectric. As to these electrons their current contribution to thedielectric surface szero. Another factor in limiting the positiveequilibrium voltage of the dielectric may be the capture, at lowincident velocity, of secondary electrons from member 62.

Beyond point b, the net current to the dielectric surface rapidlydecreases as more and more secondary emission electrons return to theirpoint of origin. With a rather small potential increase beyond point bthe current curve again crosses the zero line at a point p where thenumber of completely escaping secondary electrons just balance thenumber of primary electrons absorbed. If the potential is againincreased slightly beyond point p the current curve goes downwardly intothe negative region until a point d is reached where substantially allthe secondary electrons emitted by the dielectric surface are recapturedby the surface. Beyond point d, the current curve levels olf in thenegative region, the value of the current curve being equal to the valueof primary electron beam current impinging on the dielectric surface.

So far it has been assumed that the primary electron beam plays upon thesurface of the elemental dielectric portion for just a momentaryinterval; consequently the current conditions heretofore portrayed havebeen characteristic only of this momentary interval. It will now benecessary to consider the effect of the electron beam upon thedielectric portion for a longer period of time.

Dielectric materials are characteristically insulating materials throughwhich conduction currents will not flow. When, therefore, by thebombardment of a beam, electrons are gained by or lost from the surfaceof a dielectric portion, the resulting electrical u'nbalance cannot berestored by an offsetting current flow to or from the conducting member.The effect of an electron 'gain or loss upon the dielectric surface isthus quasi-permanent, manifesting itself respectively as a negative orpositive charge and an accompanying negative and positive potential uponthe dielectric. Further if the exposure of the dielectric surface to anelectron beam is 'a continuous one, the acquisition of charge andpotential upon the dielectric surface due to gain or loss of electrons,becomes a cumulative process, each incremental charrge deposit of agiven sense causing an incremental change in 'potential' of the surfacein the same sense which, by its effect on the beam, in turn increases inthe same sense the next incremental cha'rge deposit on the dielectricportion. Obviously because of this accrual feature characteristic of theinteraction between the dielectric surface and the beam, ultimately thesurface will be driven to a state of equilibrium with the beam such thatthe net electron current is zero.

Referring again specifically to Figure 4, assume that an isolatedelemental dielectric portion is given an initial potential between h ando. The net current to the dielectric surface in this region is negativein character representing an absorption of electrons by the surface.Exposure to the beam for a first interval of time thus producesV anegative charge and accompanying negative potential on the surface. Thenegative potential once established reduces the velocity of bombardingprimary electrons, cutting down the number of secondary electronsemitted. As a result, duringv a second interval of time, under exposureto the electron beam, the dielectric surface accrues an even largernegative charge and accompanying negative potential. The sequence justdescribed occurs continuously throughout a span of time with the resultthat the dielectric surface is driven downwardly in potential as shownby the arrow S1 until point h is reached. At point h the dielectricsurface is substantially at the same potential as the source ofbombarding electrons. No electrons will reach the surface from theelectron source, with the result that at the surface there will be noelectron loss or gain. Point h therefore represents a state ofequilibrium of the elemental dielectric portionl with the electron beam.Very minute currents from positive ion bombardment have not previouslybeen considered, but they restrain the dielectric from going morenegative than point h.

In a similar manner where the elemental dielectric portion is given aninitial potential between o and p, under continued exposure of theelectron beam, positive charge and accompanying positive potential willbuild up on the dielectric surface. The dielectric surface will thus bedriven upwardly in potential as shown by arrows S2 and S3 until point pis reached. At point p the voltage Vp between the elemental dielectricportion and the source of electrons is high,A typicallyv about two voltsabove the potential of the conducting member Vc. Primary electrons fromthe source thus will strike the surface at a high velocity, beingabsorbed thereby, and producing' by their collision with the surface alarge number of secondary electrons. 0f the secondary electrons emitted,however, only that number equal to the number of irnpinging primaryelectrons will escape to the higher potential collector electrodes 58 or59. The excess of emitted secondary electrons will be recaptured by thedielectric surface because of the suppressive effect exerted by lowerpotential conducting member 62 upon electron escape. Capture at lowincident velocity of secondaries from member 62 may also depress thepotential of the dielectric. Above point p, the surface would be urgedback towards point p as shown by arrowv S4, by recapture of an excessivenumber of 'secondarily emitted electrons. Point 'p therefore representsanother state of equilibrium of theA elemental dielectric portion withthe electron beam.

Byrway of summary it can be said that in an electrical discharge'devic'eof the type described, the surface of an elemental dielectric portionunder continued exposure' to an electron beam will be driven to one oftwo stable potentials, the alternative state assumed having the samesense as the initial voltage of Vthe dielectric portion,l

measured from a midway critical potential. y

Turning now to the practical application of the phenomenon 'outlinedabove,V Figures 5 and 6 will be considered. FigureS graphically portraysthe current characteristics of a dielectric portionV under the action.of an adjustable potential holding beam. Figure 6 pictorially @essererepresents electrical conditions at a target fa'ce subjected to thisholding beam.

Referring to Figure in more detail, the abscissa of the graph representspotentials, measured (in contrast to Figure 4) with reference to theconducting member of the target. The ordinate of Figure 5 represents netcurrent to the dielectric surface in a similar manner to the ordinate ofFigure 4. Figures 9, and 1l will use the same conventions.

Assume now that a voltage V113 for the holding cathode 26 has beenselected which voltage V113 is known to produce unlimited persistence.Assume further that the target face has been erased in such a fashionthat all the elemental dielectric portions of the face have dropped to apotential at the point h3. A positive-writing beam, intensity modulatedby amplitude gradation information signals is next caused to scan thesurface of the target. As a result individual dielectric portions areraised to various potentials. The potentials of individual dielectricportions are thus scattered in a range fromV point 113 to point p.Subsequent to the traverse of the writing beam, the entire target faceis ooded, as may be seen in Figure l, with holding beam electrodes.Under the exposure of the holding beam at voltage V113, portions ofdielectric having a potential to the left of point o3 will be drivendown in potential to point h3. Conversely portions of dielectric to theright of o3 rise upward in potential to point p. The separate dielectricportions having reached these states, the pattern so established iscontinuously regenerated by the holding beam, since any drift from thetwo states induces an immediate selfrestoring action. By the medium of aholding beam therefore, a two potential patchwork may be unlimitedlypreserved even in the presence of a reading beam whose action tends todeteriorate the patchwork.

Conditions at the target face for unlimited persistence of the twopotential patchwork are pictorially represented in section 6b of Figure6. For Figure 6 as a whole, numbers 62b designate individual Wires of amesh type target and numbers 63h designate masses of dielectric in crosssection view, which masses within the mesh interstices form dielectriccells. By means to be later more fully described, the surfaces ofportions 70 of the cells 63h are negatively charged and the surfaces ofother portions 71 are positively charged. Secondary emission from 6211is not shown in Figure 6.

In the section 6b of Figure 6, electrons emitted from the holdingcathode `and following path 72, strike wire 6211l of the conductingmember with a velocity of V113 electron volts. Since the surface ofportion 70 is at a potential corresponding to the point 113 in Figure 5,electrons following paths 73 on entering the region of the target facewill be repelled away from portion 70. Portion 70 in Figure 6b thusremains at point h3, neither gaining nor losing electrons. Primaryelectrons following paths 74 will be attracted ,to portion 71 having apotential at point p, the electrons striking the surface of portion 71at high velocity and being absorbed therein. As a result of thecollision between the primary elecirons and the surface of portion 71 aconsiderable number of electrons will be secondarily emitted fromportion 7l, the secondary electrons following paths 75 as shown. Ofthese secondary electrons following paths 7S a certain fraction willescape to collector electrodes 58 and 59, shown in Figure 1, thisfraction of escaping electrons being just suicient in amount to balancethe number of primary electrons absorbed by the surface of portion 71.The remainder of the emitted secondary electrons will be recaptured byportion 71 as shown by dotted path 76. Portion 71 therefore remains atpoint p, neither gaining nor losing electrons. The informationrepresented by the charges on 70 and 71 is permanently retained.

l. Il

4 electrons.

Returning .to a; *consideration of Figure 5 it would appear from theabove discussion `that the voltage of the holding cathode 26 shown inFigure l could be altered by variable voltage supply 28 within fairlywide, albeit reasonable limits, with an indefinitely prolonged holdingaction still resulting. lf for example the voltage of the holdingcathode is changed from point 113 to point h1 the previously outlinedsequence of events seems to apply. as properly in the latter case as inthe former. The above discussion however ignores the interactionsbetween portions 70 and 71, which interactions are significant aslimiting factors on the practical range of variance of the holdingcathode voltage productive of unlimited persistence.

If the voltage of the holding cathode is adjusted down- Wardly to pointh1'(i. e. there is an increase in potential between the holding cathodeand the conducting member) an occurrence known as positive spreadresults.

kConditions amicable to positive spread are shown in section 6c ofFigure 6. Portion 71 is still at the potential of point p of FigureS'but portion 70 is now at the low potential of h1. As a result a ratherstrong electric lield exists, running between portions70 and 71. Thiscross field has a iirstieffect of deflecting electrons following path 77away from the outlying surface of portion 70, upon the margin of whichthe electrons normally would replenish the negative charge, and towardsan adjacent part of portion 71 where the electrons build up the positivecharge. If the negative charge upon the margin of portion 70 is notreplenished, the voltage of the margin drifts upwards due to theoccasional arrival of positive ions which deposit positive charge. Thussince positive charge is gained on one side and negative charge lost onthe other the boundary between portions 70 and 71 creeps towards thelatter.

As a second effect, since a higher holding voltage exists, namely V111,the voltage gradient of the electric lield across the boundary betweenportions 70 and 71 is increased as compared to Figure 6b. If thegradient becomes strong enough, in the vicinity of the boundary, by amechanism known to the artv as eld emission, electrons, representing anegative charge loss, will be pulled out of the surface of portion 70,as shown by path 78, and will be transferred over to portion 71. A shiftof the boundary line at the expense of portion 70 ensues. i Converselyif in Figure 5, the voltage of the holding cathode is adjusted upwardsto point 115, an opposite condition to positive spread, namely negativespread, will be produced. The nut-:chanismV of negative spread may bestbe understood by reference to section 6a of Figure 6. Primary electronsin the holding beam as they bombard the target are moving with therather slow velocity of V electron volts, thus producing, as comparedwith the situation in 6b a low number of secondarily emitted Whereelectrons following path 79 fall upon the margin of portion 71 nearportion '70, the negative charges on portion 70 suppress secondaryemission to the extent that insufficient numbers of secondary electronsescape to maintain positive equilibrium. These primary electronsfollowing path 79, therefore, produce a negative rather than a positivecharge upon the margin of dielectric portion 71 adjacent to portion '70.The area of negatively charged surface is thus gradually enlarged,portions 70 encroaching upon portions 71.

Because of the occurrence of positive and negative spread, for unlimitedpersistence the satisfactory range ot variance of holding cathodevoltage is rather narrowly prescribed. A proper holding voltage forunlimited persistence is represented, in Figure 5 by point 113, thecurrent curve for which contains approximately equal areas under thecurve for the negative and positive regions of net current tothe surfaceof the dielectric. For a variance of approximately plus or minus 5% frompoint h3, as represented by points 114 and h2, unlimited persistence@essere may still be maintained. BeyondY the limits demarcated by pointsh4 and h2, the information pattern on the target face deteriorates overa period 'of time. I'he rate of deterioration, however, may be'.regulated in accordance with the adjustment of the holding' cathodevoltage In certain applications icontrollable persistence of this' sortmay be desirable as for example where successive radar echoes areretained upon a screen to indicate the velocity of an observed target.

Referring again to Figure l, commonly for operation of the presentlydescribed information storage device, the holding beam is maintained ina' continuous' on condition by a direct connection of holding gridv 52to holding cathode 26 through switch 5'6. For certain modes ofoperation, however it may be desirable to turn off the holding beam.This may be accomplished by the connection of grid 52, by means ofswitch 56, to holding cathode 26 through bias supply 57.

The above decsribed' holding phenomenon is characteristic both ofsystems capable of storing binary information only and of a system whichadditionally may indenitely store amplitude gradation information. It isnow necessary to consider the mechanism of the present invention bywhich amplitude gradation information may be utilized.

Turning specifically to the features which permit retention andreproduction of amplitude gradation information signals, Figure 7 is agreatly enlarged elevation, viewed from the cathode end of the tube of asingle dielectric cell 63b Within aninterstice of the mesh type target.For the purposes of illustration the body of the dielectric cell 63b maybe thought to be divided into a set of elemental dielectric portions 80,81`and 82, each portion being a Volume ha'ving' one surface contiguousto wires 6217 and vone surface a subdivision of the entire exposedsurface of the dielectric cell. The mode of division selected may bemore clearly seen by reference to Figure 8. Portions 80, 81 and 82 areso selected that their suriicial areas are equal.

It will be obvious to those familiar with the art that each of portions80, 81 and 82 may be considered as anv elemental capacitor, wires 62band the exposed surface of the portion being analogous to the plates ofa condenser separated by a layer of dielectric. Since the exposedsurfaces of portions 80, 81v `and 82 have equal areas, the values ofcapacitance for portions 80, 81 and 82 are dependent only on and areinversely proportional to the mean spacings mb, mil and" mi2,respectively, between the opposed surfaces of the portion. Portion 82therefore is equivalent to a smaller elemental capacitor than portino 81which in turn is equivalent to a smaller elemental capacitor thanportion 80. For the purposes of the invention it is Adesirable that thesurface capacity per unit area vary relatively continuously over a widerange. In Figure 8, the surface capacity of portions 80, 81 and 82varies over a range of about 6 to l. Further subdivision would show amuch greater range in high capacity adjacent 62b.

Assume that in Figure 7, dielectric cell 63b is initially in anerasedcondition. If a. writing beam is traversing the target face passesover cell 63B, the writing beam will dwell upon the cell for a shortinterval of time t determined by the equation amount of simple charge Qwill be deposited on the surfaces of portions 80, 81 and 82 inaccordance with the relation Q'=K1]t where I is the instantaneouscurrent density of the writing beamand'Klkis Va constant. lecollectinglthe fundamental equation for a capacitor where V standsrfor potentialand C for capacitance, it is seen that since portions 8i), 81 and' 82have diierent capacita-nce values, although the quantity of simplecharge deposited on the surface of each portion is the same, the simplepotential exhibited by the various portions will differ. Since t forusual operation is a constant, the simple potential, immediately .afterWriting, of any dielectric portion may be expressed in the form V=KTFrom the above expression it is evident that Within each of a pluralityof cells 63b, the mean potential of the group of elementalv capacitors80, 81 and 82 will be proportional to I, butthat the deviation of anyindividual elemental capacitor from the mean potential depends on C, thevalue of its capacitance. High capacity portion 8b will undergo theleast shift from erased potential, 81 and 82 ,progressively greatershifts.

It Will be apparent to one familiar with the art that the division ofcell 63h into merely three elemental capacitor portions, is a crudeapproximation to reality, and that to properly accord` with fact eachdielectric cell should be considered subdivided into many elementalcapacitors, each having a diiferent capacitance value.

In the region of proportional operation of the invention, the chargingcurrent, which may be a current modus lated electron beam, charges somebut not all dielectric portions from the erase-d potential to a valuebeyond the critical' potential. The unmodulated writing beam wouldcharge a substantial fraction of the target dielectric area, in somecases half, beyond' critical. Should the positive or negative'modulation peaks reach values which charge all or none of thedielectric area beyond the critical value, such a condition wouldrepresent peak clipping andresult in signal distortion.

Under the mean or unmodulated value of charging current, about half thetarget area will, under the holding beam, be converted to the otherequilibrium potential. The resulting mean potential will correspond tothat produced by the writing beam, and will be equal thereto where thecritical potential is midway between the two equilibrium potentials.

Under positive peak modulation, an initially negative target area willbe almost all charged above critical potential, and its mean potentialresulting from holding beam operation will be raised considerably abovethat produced by thev writing operation. Correspondingly, negative peakmodulation charging of a negative target area will charge only a smallfraction of the target area (K2 is a constant) above critical potential,and the resulting mean potential of the area under the holding beam willbe lower than that produced by the writing operation.

While the mean values of target area potential arenot identical, afterholding beam operation, with those pro'L duced by the Writing operation,their relations are sub-k stantially linear and the final patternrepresents the imposed signal as .amplified in modulation amplitude.rhis.

inherent amplification is a ve'r'y desirable fact-or.V

It will be appreciated by those familiar with the art that the analysiscentering'a'bout -Figure 7 is highly simplied and is only utilized toconvey in a general Way an understanding of the operation of the device.The' applicants invention'of course is independent of any theoreticalprinciples which may be applied to explain its.

operation.

The plate type target shown in Figures l and 2 manifestly presents areaswhose surface capacity varies over a substantial range due to dielectricthickness and to the Varying. effectV of dielectric aggregate area onfringe ca-V l pacity. The latter, normally effective at the edge oflarge condensers, is operative throughout the areas of the smalldielectric aggregates.

In both type of target surface shown, it might be desirable for somepurposes that the dielectric-surface area present relatively uniformdistribution of area over the range of.surface -capacity employed in theproportional writing operation. It is suflicient for most applicationsthat the surface present a substantially continuous distribution of areawith capacity. It will be understood, of course, that should a targetlack in any particular unit area an appreciable fractional area having asurface capacity per unit area lying intermediate of its intendedcapacity variation range, writing signal amplitudes which would justcharge such areas beyond the critical holding potential will bedistorted.

Having described the mechanism by which half tone information items whenonce written may be indefinitely retained upon the target face, thevarious modes of operation of the tube may be discussed by which theinformation is initially impressed upon the sensitive storage surface.Depending on the operative parameters the writing action may be eitherof the negative or positive variety.

Employment of negative writing requires Vthe preliminary step ofproducing a uniform background by driving all the dielectric portions ofa selected area to Vthe upper stable limit of potential. This step maybe accomplished by positive erasure in a way later described. The areais then scanned by the writing beam, the constituent electrons of thebeam having a slow enough velocity to result in deposition of negativecharges on the dielectric portions. Since the dielectric portionsinitially Yare at the upper stable limit of potential, suiciently slowelectrons can only be obtained by adjusting writing cathode 33, shown inFigure l, upward in potential until the voltage between the cathode andthe background is less than V0, the critical voltage. When writing isaccomplished, information items will be manifested as unit areas havingnegative potentials relative to the background.

Conversely when positive writing is employed, the elemental dielectricportions of a selected area of the target face by negative erasure areuniformly driven to produce a background at the lower stable limit ofpotential. The writing cathode 33 hence must be adjusted downward inpotential until the voltage between cathode 33 and the backgroundexceeds Vo. The constituent electrons of lthe writing beam thus willhave a high enough velocity to positively charge the dielectric portionsas the beam scans the selected area. In the case of positive writingtherefore, items of information will be manifested as unit areas at apositive potential relative to the background.

Whether negative or positive writing is utilized, translation, ofreceived information signals into charges on the target face isaccomplished by impressing the signals in the form of voltages on grid33 which controls the current density of the writing beam.

Usually it is desirable to maintain the holding beam in an on conditionduring the writing process, since by doing so the background to thewriting will not drift in potential. Continuous operation of the holdingbeam, however introduces a new problem. Obviously when either positiveor negative writing is used, at least some of the newly chargeddielectric portions must be driven from their original potential to apoint beyond the cross over voltage for the holding beam. Otherwise uponcessation of the writing current, all the portions within a unit areawould lapse back to their original potential and the information carriedby the unit would be utterly eifaced. The holding beam itself howevermay prevent a movement of the dielectric portions beyond its own crossover voltage, and for the following reason. Under bombardment by boththe holding and the writing beams a given dielectric portion'has acurrent curve which is the sum of the two curves of the same portionproduced separately by the two beams. .The summation curve deviates bydips and rises from the writing curve, and if a dip or rise shouldchance to cross the zero line, a spurious stable state will appear atthis point. With such a spurious point dielectric portions influenced bythe writing beam will charge in contrast to the background until theyreach the point where theyY will stick, unable to change potentialfurther to pass beyond the cross over voltage of the holding beam. As aresult, upon cessation of the writing beam and under holding beamexposure these portions will slidel back to the potential of thebackground whence they came.

In order therefore to produce effective writing, the writing beam musthave suicient current density and voltage to both impress a contrastcharge against the background and to override the influence of theholding beam, which causes the summation current curve to cross the zeroline at an additional point. Modes of writing now will be describedwhich satisfy these requisites.

Figure 9 shows operating parameters for one form of negative writing.Dielectric portions of the target initially exist at the potential ofpoint p. The dot-dash curve lz4-a4-04-b4-p represents the current curvefor a dielectric portion under the action of the holding beam utilized.The voltage between the conducting member and the holding cathode isadjusted -to a value 'Vh4 which, as may be seen from Figure 5,represents a limit for unlimited persistence holding, voltages of lessermagnitude than Vh4 causing negative spread. With theholding voltage Vh.;utilized the distance between points h4 and o4 measured along thehorizontal -ordinate is greater than the distance between o4 and p.

The full line curve passing through points he, as, r and o6 representsthe current curve of dielectric portion under bombardment by the writingbeam. The writing curve is shown as having considerably greaterdisplacement along the vertical ordinate than the holding curve sincethe writing beam has a much greater current density than the holdingbeam, typically 6000 microamperes per square inch as compared to 10. Thesummation curve is represented by the line a4fgr.

Since the critical voltage is dependent only on the nature of thedielectric material, the distances (measured along the horizontalordinate) of h4 to o4 and h6 to o8 are the same. As mentioned heretoforethe distance o4 to p is less than h4 to o4; therefore it is less than h6to o6. Point h6, however is located just below point o4 in potential. Asa result dielectric portions existing at the stable potential p withrespect to the holding beam, will appear to the writing beam as ifexisting at a point r which is in the negative region of the writingcurve. Hence under bombardment by the writing beam the dielectricportions by the absorption of electrons will be negatively charged. As aresult of the accrual feature hitherto described, all the portions in aunit area will differentially drop in potential along the dotted linesummation curve. Upon cessation of the writing beam, those portionswhich have dropped below o4 will be driven to point h4 by the holdingbeam, and those which have not dropped so far will rise again to pointp.

lt will be noted that if point h6 is shifted an extent to the left ofits presently shown position, the background dielectric portionsexisting at the potential of the point p will not appear to thewriting-beam as being in the negative region of the writing curve, andno negative writing can occur. Conversely if point h6 is shifted fromits presently shown position to the right of point o4, the summationcurve will cross the zero line to the right of point o4. This point ofzero current for the summation represents a spurious state of stabilitywhich prevents eiective writing for the reasons hitherto described.

Referring to Figure lO a mode of negative writing is disclosed in whichthe direct voltage of4 theholding cath- 'ode need not be adjusted awaytrom the value Vh3, which Voltage as shown by Figure is in the middle ofthe holding region for unlimited persistence. The current curve for adielectric portion subjected to a holding beam alone of direct voltageVhs is shown by the full line curve h303b3p. The current curve for adielectric portion subjected to the action of the writing beam alone isshown by the curve hsaro. The summation curve resulting from the actionof both the holding and the writing beams is represented by the dottedline curve oafgr. Y

lt will be noted that the summation current crosses .the zero line atpoint f, and that point f therefore represents a spurious stable state.If the holding voltage is kept at the steady value Vha, any dielectricportion originally existing at the background potential p, no matter howquickly it is negatively charged by the electrons of Ithe writing beam,will drop no further in potential, 'than the point E, at which point itremains until subjected to the holding beam alone at which time itreturns to p. Dielectric portions arrived in the region of point f,however, can be induced to further drop to point h3 if the holdingvoltage is transiently rocked upward in potential so that the holdingcurve cross over point o3 moves past point f as shown by 0.3. Atransient rocking characteristic of this sort may be imparted to theholding voltage by superposing an alternating component Vs upon thesteady negative voltage Vh3. As shown in Figure l, in practice this maybe accomplished by capacitively coupling holding Acathode 26 toalternating signal source 32 by closure of switch 31. Of course anyother fluctuating signal having an upward and downward movement may besubstituted for VB.

Returning to Figure upon application of alternating voltage Vs thelimits of shift of the holding cathode upward and downward in potentialare represented by the points h3 and h3 respectively. The current curvecorresponding to the upward limit of the holding cathode is portrayed bythe dot dash line h3 a3 03 b3 p.

As the holding voltage shifts upwardly, the cross over point o of thecurrent curve will move rightwards of point f. Upon the happening ofthis event dielectric portions at the potential of point f will appearto the electrons of the holding beam as in the negative region of theholding current curve. As a result these dielectric portions will beginto drop down in potential. Subsequent to rising to its upper limit, h3a3 o3' b3 'p, the holding curve as an entirety shifts downward inpotential. The dielectric portions to the left of point 0'3 will thenride the curve, to use an analogy, like a Surfboard onpa wave, droppingin potential all the while until the portions will fetch up at point h3.Upon removal of the alternating voltage Vs, these dielectric portions,as a result of positive ion currents, will move upwards in potential topoint h3. Y

By the employment of a shifting holding voltage it is seen that separatedielectric portions within a unit area, initially impressed with varyingnegative potentials relative to the background, may beurged to one oftwo potential limits dependent yon the quantity of charge acquired.Negative writing is thus possible without disturb ing the adjustment ofthe direct holding beam as fixed to operate in the middle of theunlimited persistence holding region.

vWith either negative or positive modes of writing, the writing speed'of the device is limited to the rate in scan at which the writing 'beamcan place sufcient charges on the dielectric portions so that afterexposure to the holding beam the written trace perceptiblycontrastsagainst the background. In its turn, the'rapidity with which a singledielectric portion can be charged depends upon the effective chargingcurrent of the writing beam. For negative writing the effective chargingcurrent equals primary beam current minus secondaryV emission current;for positive writing it is the reverse, secondary emission current minusprimary beam current. Primary beam current is Va' fixed quantity;secondary Vemission current however may vary from close to zero toseveral times the primary beam current, depending on the magnitude ofwriting voltage. Obviously, while the effective chargingcurrent fornegative writing cannot exceed the primary beam curf rent, for positivewriting, the yeffective charging current may be several times thatvalue. Positive writing therefore permits a faster writing speed thannegative writing, and it is preferable on that account.

Figure ll discloses graphically operating parameters v suitable forpositive writing. Full line haasosbgp represents the current curve for adielectric portion subjected to electrons from a holding beam adjustedin'potential holding region for unlimited persistence. Full linehflaq'oqbqp similarly represents the current of a dielectric portionssubjected to a writing beam of voltage Vhq. Under bombardment of bothbeams the summation curve of the portion is shown by dotted line fgjp.

writing is fairly obvious. Initially all dielectric portions of aselected area by negative erasure are driven to a stable backgroundpotential at the point h3. To the'positive Writing beam, however, thedielectric portions scanned will appear in the positive region of thesummation curve. As a result during the interval of time the writingbeam dwells on the dielectric portions in alunit area, the portions willbe driven upward in potential from ha, the varying capacitancesof theportions causing in which the holding beam is maintained in an onwriting voltage used, while not critically curtailed, fmustz Y be withina prescribed range. In upwards adjustment the writing voltage must bekept below thevalue where the summation curve would cross the zerocurrent linevat` an extra point thereby introducing a spurious stablestate. In downward adjustment the writing voltage must be kept above thepoint where for agiven rate of scan the electrons of the writing beamwould positively saturate all dielectric portions they bombard'duringthe period of dwell.

Figures 9, 10 and 11 have dealt with modes of writing condition duringthe action of the writing beam. If.V desired, however, writing isfeasible with the holding beam turned o as shownV in Figure 1, byconnection of grid 52 to holding cathode 26 through switch 56 and biassupply 57. kkIn such case if the dielectric surface isat a posi-k tivebackground potential, the writing cathode is adjusted upwards in voltageuntil the writing beam electrons produce negative charges upon impactwith the' dielectricY portion. j Conversely, if the dielectric surfaceis at negative background potential the writing cathode is adjusteddownwardly in voltage until the writing Ibeam electrons have sui'lcientvelocity to cause positive charges on the dielectric portions. Y

As has been mentioned heretofore the means by which a reading current isinduced is a distinctive feature of the storage tube presentlydisclosed. It will be recalled that a unit area includes within itsbounds a multitude of dielectric masses separated by conductinginterspaces, the interspaces being exposed zones of the conductingmember. It will further be recalled that by the action of the writingbeam an information unit will display a mean potential caused by theaggregative effect of numerous dielectric portions having simplepotentials distributed in value around the mean potential. This meanpotential,

standing for an information item, is linearly amplied'byf an exposure tothe holding beam when the separate di- In Figure 10the sequence ofevents producing positiveV electric portions shift to one or the otherof two stable limits.

When an information unit is traversed Vby the reading beam, primaryelectrons impinge upon the exposed interspaces of the conductingsubstance and are absorbed therein. The striking velocity of theseprimary electrons is sucient to cause secondary emission of electronsfrom the conducting substance. Of the entirety of secondary electronsinitially emitted from a given interspace, however, the number ofelectrons which inally escape is regulated by the simple potentials onadjacent dielectric portions. The dielectric portions thus control thesecondary emission current from a contiguous interspace in a manneranalogous to the way a grid controls the cathode cur rent in an ordinarytriode.

Conditions at the target face during bombardment by the reading beam arepictorially shown in Figure 12. As in Figure 6, Figure 12 represents agreatly enlarged cross section of small part of mesh type target withnumbers 62]) designating individual wires of the mesh and numbers 6317designating dielectric masses, which masses within the mesh intersticesform dielectric cells. Primary electrons of the reading beam followpaths -83 as shown. Paths of reading beam electrons which impinge uponthe dielectric masses are omitted. Paths also are not shown forelectrons which though secondarily emitted by the exposed wire surfacesdo not escape therefrom.

In section 12a of Figure l2, the entire surface of the dielectric massis negatively charged, as represented by the minus signs in portions 70and 71. The influence of portions 70 and 71 does not extend to primaryelectrons of the reading beam following paths 83. The primary electronstherefore impinge upon the wire 62b with a velocity given by the voltagebetween vthe conducting member and the'reading cathode. As a result ofthis collision the primary electrons are absorbed and secondaryelectrons are emitted. The fields associated with portions 70 and 71,however, prevent any of these emitted lsecondary electrons from escapingfrom the region of the wire; instead all of these initially emittedelectrons vultimately fall back on the wire surface, furnishing a zerocurrent component.

In section 12b of Figure l2, dielectric mass 63b is half negativelycharged as shown by portion 70 and half negatively charged as shown byportion 71. Portions 70 and 71 exert an opposite inuence upon electronssecondarily emitted from wire 62h, portion 70 suppressing escape andportion 71 encouraging it. As a resuit,` of the entirety of electronssecondarily emitted, approximately half escape as shown by paths 84.

In the section 12C of Figure l2 the entire surface of dielectric mass63b is positively charged as shown by the plus signs on portions 70 and71. Both portions 70 and 71 therefore encourage the escape of electronssecondarily emitted from wire 62h. As a result, substantially all theelectrons secondarily emitted will finally escape as shown by thegreater num-ber of paths 84.

With the reading beam voltage used, each primary electron upon collisionwith the wire surface produces on the average more than one secondaryemitted electron. It follows that if all the emitted secondaryelectronsare permitted to escape, as shown by section 12e of Figure l2, secondaryemission current will exceed primary beam current. In practice it isfound that asV an'V incident to this multiplicative effect, the usefulrange of modulation of the reading current exceeds the magnitude ofprimary beam current by an appreciable factor, typically 20%.

If the number of secondarily produced electrons escaping from a giveninterspace within a Vunit area is dependent on the simple potentials ofadjacent dielectric portions, it is obvious that the number of electronsescaping from a unit area as a whole will be proportional to` the meanpotential of the information unit, the vmean potential being measuredrelative to the bakgIQurid pos tential. Since the mean potential of aunit area typies 'an amplitude gradation information item, the quantity-of escaping, secondarily produced electrons also is a measure oftheinformation item impressed upon the unit area. Now, as it was earlierdiscussed, primary electrons --absorbed by the conducting member willproduce a negative current component flowing to the conducting .memberfrom impedance 65 and secondary electrons es- '.caping from theconducting member cause a positive cur- .rent component owing from theconducting member to the impedance 65. During a scan in which successiveinformation units are traversed by the reading beam, the negativecurrent component remains the same, equalling in magnitude the number ofreading beam electrons absorbed by the conducting interspaces of theunit. The negative current component thus furnishes a background levelsignifying zero information. Concurrently, the positive component'of thecurrent fluctuates Vin accordance with the amplitude gradations of themeanpotentials on the unit areas, instantaneously equalling in magnitudethe number of escaping secondary electrons from the unit area which atthat timerthe reading beam bombards. Thus when an information item iselicited by the reading beam the item will be manifested as the positivecomponent of the net current owing from the conducting member toimpedance 65.

In the embodiment of the invention disclosed in Figure 1, the netcurrent flowing from conducting member 62 passes through impedance 65,generating a voltage across the terminals of the same. From the netsignal thus produced, by the utilization of a voltage level transposer,an output potential may be obtained linearly related to the positivecurrent component. ln the present case the voltage level transposertak-es the form of a capacitor 67 coupling the Vungrounded terminal ofimpedance 65 to utilizationV device 56.

It will be noted that the reading principle of operation requires onlythat the reading beam bombard the interspaces of a unit area. Nobombardment of the interspersed dielectric masses is necessary, sincethe dielectric portions can perform their regulating function solely byvirtue of the enduring charges and potentials regenerated on them by theholding beam. Incidental impingement of electrons on the dielectricmasses is unavoidable, however, as the reading beam dwells on the unitarea. Reading beam voltages outside the holding voltage range would thustend to destroy the information pattern regenerated by the holding beam.Destruction of this sort in turn curtails the number of effectivereading repetitions. It is desirable therefore to adjust the voltage ofthe reading cathode to a value similar to the Vvoltage of the holdingcathode. By such adjustment the reading beam in acting on the dielectricmasses will supplement the action of the holding beam.

Although having the same voltages, both reading and holding beams arepreferably employed simultaneously. Under these conditions, when a givenunit area is scrutinized by the reading beam, although both holding beamand reading beam electrons move toward the unit area with the samevelocity, the number of electrons impinging on the scrutinized unit areais much greater than for surrounding unit areas subject only tobombardment by the holding beam. This greater density of the electronconcentration striking a pin-pointed unit area results in the productionof a differential current indicative of the information item carried bythe unit area.

Before new writing may be impressed upon the target face, the oldpotential patchwork must be cleared away lwhich process is callederasure. Erasure is either positive or negative depending on whethernegative or positive writing respectfully is to be subsequentlyutilized. In the erasing operation al-l of the elemental dielectricportions of the entire target surface or a selected area thereof aredriven to the upper or lower stable potential limit, as the case may be,the uniform potential pattern assae've 21 'thus produced representing abackground .clear of information items. the unit areas by the 4writingbeam will contrast again-st this zero information background.

Both positive and negative erasure may be accomplished by the holdingbeam. Where positive erasure is desired the voltage of the holdingcathode is adjusted downwardly until the most negatively chargeddielectric portions of the old potential patchwork appear in thepositive current region of the holding curve. The negatively chargeddielectric vportions are thereby driven to the upper stable limit atpotential p. Dielectric portions already at point p are unaffected bythe higher voltage of the holding beam since any trend towards apotential rise, caused by increased secondary emission, is counteractedby the suppressor grid effect of conducting member 62. f l

Negative erasure is an inherently slower process since in order to causenegative current at the surfaces of the dielectric portions charged, inthe old patchwork, to the positive potential p, it is necessary toadjust the voltage of the holding cathodeupward of the chosen lowerstable limit of potential. If nothing else is done, the dielectricportions initially at p will shift down to the voltage of the holdingcathode and will drop no more. A further drop in voltage to the lowerstable limit of potential may be induced, however, in these portions ifthe holding cathode voltage is now adjusted downward to the lower stablelimit. The potentials vof the more positively charged dielectricportions follow the holding cathode in its drop.

It will be readily appreciated by those familiar with the art that theinformation storage device presently disclosed is nowise limited to themodes of operation described above. For example the tube may be operatedin a manner permitting simultaneous -writing andreading on the surfaceof the target. When the Writing beam bombards the dielectric portions ofa unit area, electrons of the beam will incidentally strike theinterspatial zones of the conducting member. A secondary. emissioncurrent thereby is produ-ced which normally would mask a simultaneouslyproduced reading current. lf however, as shown in Figure l, two-wayswitch 50 is operated to disconnect grid 43 from reading cathode 27 andto couple grid 43 to radio frequency voltage source 51, the reading beamprimary current will be varied at the radio frequency signal rate. As aVresult information items elicited by the reading beam appear asamplitude modulations upon a radio frequency carrier. For simultaneousread and write, filter 69 is connected to the junction of capacitor 67and device 66 by closure of yswitch 58. Since filter 69 is tuned to thefrequency of radio frequency voltage source Sl, lter 69 offers a highimpedance to the reading signal modulated carrier but a low impedance toany other current. Reading signals may thus be separated from theconcurrent masking signals incidentally generated by the writing beam.

Is is also obvious that if a focussed beam (of either the reading or thewriting gun) is lirst adjusted to a voltage in the normal holding regionand then continuously scans the target face, the focussed beam willregenerate the information pattern as effectively as the diffusedholding beam hitherto described. Although each unit arca is only subjectto the action of the focussed beam for a small fraction of the scanningcycle,l this fact is compensated for by the much greater current densityof the focussed beam as compared to the diffused beam. Employment of thefocussed beam has the advantage of giving the operator the option ofutilizing selected areas rather than the entire target face as aninformation storage surface.

It follows from the above that the diffused holding beam gun may beentirely eliminated from the storage tube assembly, the beams from theremaining writing and reading guns, being utilized to perform all thenecessary functions relative to information handling. In such aPotentials subsequently generated on 22 t two gun tube, a preferablemode of p sign only the single usual function of writing to the writingbeam. The reading beam however is assigned .a multiplicity of functions.The reading beam if operated in continuous scansion at a voltage inthenormal holding region will both preserve the information pattern andproduce a reading current. According to the wishes of the operator theperiod of operation for the reading ,beam may either exclude or includethe writing interval.V In the latter case, of course the; voltage of thewriting beam must be of a magnitude to override the holding effect ofthe reading beam. Erasure is preferably accomplished by a' continuousscansion of the reading beam while adjust/edl to an erasure voltage asdescribed above. Ifsimultaneous reading and writing is desired'thedensity ofthe readingbeam is radio frequency modulated asdescribed'above.A

Further by the employment of a time sharing principle;l` the informationstorage devices may be satisfactorily operated with just one focussedbeam gun, the other two:

guns being eliminated from the assembly. -In such a one gun tube,successive writing, holding plusreading, andv erasure voltages aresuccessively impressed for intervals upon the cathode of the single gun.Simultaneous reading and writing or the maintenance of a holding effectwhile writing, iny this case, of coursefi's it feasible.4Manyapplications of the Y presently disclosed device will suggestthemselves to those familiar with ,theart.l For example, the informationtube may be employed for; the storage and presentation of signalsreceived by radarV and sonar systems, for the preservation of televisionimages and as an information accumulation unit ingelec.- troniccomputers.

Asiwill be understood the subject invention is'caprableV of manyembodiments and applications. The specific embodiments disclosed in thisapplication are exemplary only and are not submitted for thenpurpose ofdefining the limits ofthe invention.

The invention described herein may be manufactured face portions beingrelatively small with respect tosaid elemental reading area, saidsubstantially discrete dielecv tric surface portions having surfacecapacitydistributed throughout a substantial continuous range, saiddielectric portions being distributed over the target surface with atleast random uniformity in a density presenting in any elemental readingarea said substantial range of surface capacity. f

2. The structure of claim 1 further including target charging meansoperative on a target charge pattern area selectively in dependency onthe surface capacity of the dielectric portions thereof to charge saidportions to potentials distributed through a substantial continuousrange, said range in every reading area over the pattern area includinga .common intermediate potential, and cathode means operative to supplyelectrons to the pattern .area from a potential negative of said commonintermediate potential by ank amount equal the critical electron voltagefor unitary secondary emission.

3. A cathode ray information storage device comprising, in combination,an evacuated envelope, a collectork p electrode and rst, second andthird electron beam K sources disposedV within said envelope, separatespot focussing and deecting means for said iirst and second beams,diffusing means Vfor said third beam, means for intensity modulatingsaid trst beam, an information storperati'on Ito as-f age target Withinsaid envelope comprising an electrically conductive member and anaggregate of dielectric material adherent thereto to form therewith asource confronting facing including a plurality of minute sizedsubstantially dielectric material exposures intermingled with exposuresof said member, said dielectric material exposures occurring insubstantial concentration for each target area of focussed electron beamspot dimensions.

A 4. A cathode ray information storage device as in claim 3 in which theconducting member has a continuous surface confronting said sources andthe aggregate of dielectric material comprises a plurality of mutuallyspaced dielectric particles of random sizes adherent to said surface. v

5. VA cathode ray information storage device as in claim 3 Vin which theconducting member is a mesh and the aggregate, of dielectric material isadherent to the mesh to format least partial closures of the meshinterstices.

6. A cathode ray information storage device compris` ing, incombination, an evacuated envelope, a collector electrode and first,second and third electron beam sources disposed within said envelope,separate spot focussing and deecting means for said first and secondbeams, diffusing means for said third beam, means for intensitymodulating said rst beam, an information storage target within saidenvelope comprising anV electrically conductive member and an aggregateof dielectric material'adherent theret'oto form therewith aa sourceconfronting facing including a plurality of minute sized substantiallydiscrete dielectric material exposures in termingled with exposures ofsaid member, said dielectric material exposures occurring in substantialconcentration for each target area of:

focussed electron beam spot dimensions, and anV output impedance coupledbetween said member and said collector electrode.

7. A cathode ray information storage device as in claim 6 further4characterized by means for intensity modulating the second electronbeam with an alternating carrier `signal and control means responsive toand operable in ac- 24 cordance with said signal, said control meansbeing coupled to the output impedance to control the output thereacross.

S. A cathode ray information storage device as in claim 6 furthercharacterized by means for selectively modulating the intensity of thethird electron beam with a uctuating signal.

9. In a` charge storage device, selectively exploring charge readingmeans operative to detect the charge on a predetermined elementalreading area, and a charge storage target having a face Vpositionedoperatively with respect to the charge reading means comprisingsubstantially discrete dielectric surface portions and conductivesurface portions, said dielectric surface portions being distributedover the target with at least random uniformity in a density presentingin any elemental reading area a multiplicity of dielectric Vsurfaceportions and a multiplicity of conductive surface portions, saiddielectric surface portions having a surface capacity distributedthroughout a substantially continuous range and distributed to presentsaid substantially continuous range of surface capacity in any elementalreading area.

References Cited in thel tile of this patent UNITED STATES PATENTS2,149,977 Morton Mar. 7, 1939 2,193,101 Knoll Mar. 12, 1940 2,324,504 YIams et al July 20, 1943 2,373,396 Hefele Apr. 10, 1945 2,415,842VLOliver Feb. 18, 1947 2,500,633 Edwards Mar. 14, 1950 2,535,817 SkellettDec. 26, 1950 2,549,072 Epstein Apr. 17, 1951 2,594,740 DeForest et alApr. 29, 1952 2,640,162 Espenchied May 26, 1953 2,687,492 Szegho et al.Aug. 24,1954 2,757,233 Webley July 31, 1956 2,777,060 Waters Ian. 8,1957

